18 & 19" on the same car?

The profile is a percentage of the width, so a 40 profile means the tyre wall height is 40% of the width. Therefor to achieve the same diameter the profile on the 18” wheels must be higher.

 

So your saying that the tyre's need to match the profile for staggered 18's on F34? i.e 50 front & 45 rear as its only the profile that differs tyre size is the same

 

Yes, so the smaller wheel diameter should have a higher profile to keep the same rolling diameter. If you have 40 profile on both 18’s and 19’s for example the rolling diameter of the 18’s will be quite a bit smaller. You can’t have that on X drive cars.

 

Ok understand but if I fit all 4 18" wheels then it should be ok?

 

Ok so based on this:

Front 225/45/19 8J x19 ET 30 19x45% = 8.5
Rear 255/40/19 9J x 19 ET 42 19x40% = 7.6

0.9 difference

My 18's are:
Front 225/45/18 8J ET 34 18x45% = 8.1
Rear 255/40/18 81/2 J ET 47 18x40% = 7.2

0.9 difference

18's with the correct profile & ET
Front 225/50/18 ET30 18x50% = 9
Rear 255/45/18 ET42 18x45% = 8.1

0.9 difference

So using kleynie's calculation of % of width for correct diameter gives me the above so now what?? As I am totally confused, even running a mix of 18 on front and 19 on rear gives me less at 0.5

 

It's not percentage of wheel diameter, but the percentage of tyre width.

So, 225/45/19 would give a tyre height of 101.25mm (45% of 225)

255/40/19 would give a tyre height of 102mm

The rolling circumference for the 225 tyre would be 2152.3mm, for the 255 tyre it would be 2157mm. The difference being approx 0.3% - well within the 2% tolerance.

Have you checked out willtheyfit.com?

 

Ok so based on that the 18's would be fine as the tyre height is the same am I correct

Front 225/45/18 8J ET 34 tyre height 101.25
Rear 255/40/18 81/2 J ET 47 tyre height 102.0

The correct profile is

Front 225/50/18 tyre height 112.5
Rear 255/45/18 tyre height 114.75

Still can't see the issue with my 18's as ride height remains the same help!!!!!!!!

 

I thought that’s what I said, but maybe not so well?

 

The issue is with mixing the 18’s and 19 tyre sizes you mentioned

 

I understand that but if I change 2 on the same axle then things stay the same, sorry really really confused

 

Because you have X drive you need to change all four to keep them all the same rolling diameter or you will goose your transfer box

 

As Ian says the percentage difference in rolling circumference must be within 2% on the same axle and also between the front and back axle. So mixing 18s and 19s can invite trouble unless you're careful with tyre sizes.

You did, but I felt it needed saying again

 

Ok so fitting all 4 18's (which was my original idea) will be fine as everything stays the same........am I correct (pls say yes )

 

Edit: yes, if they have the correctly sized tyres.

 

Cool thank you yes tyres are the same size on my 18's purchased as the 19"s currently fitted

 

If you're ever unsure, check out willtheyfit.com, aside from offset, one key measurement it helpfully lists is the % difference in rolling circumference (speedo error they call it) it's this % that needs to be as low as possible - under 2% to minimise transfer box issues.

 

Fine to use an 18 or 19" wheel set. Be it the 18" wheel set is 'undersized' and speedo will be out. It's also the problem with mixing, as you don't want the same profiles and sections.

Peter

 

Thanks I originally purchased to use as a set (as they have TPMS fitted) and were a bargain @ £380 for the set delivered with matching bridgestone RF tyres all above 5mm tread, but wondered if I could just use 2 on the same axle if needed but obviously I can't.

Speedo will be out by 3.84% so @ 30mph it will be 31.15mph

 

I did use that website but only for checking fitment for the whole set of 4 not mix and match

 

@bishbosh how do you check the tolerance from front to back with staggered setup on that website? Get the whole same axle thing but not sure on the rest??